1. Field of the Invention
The present invention relates to a cutting type tap and a plastic working type tap which are suitable for tapping of a cored hole or the like.
2. Description of the Related Art
Formation of an internal thread has so far been performed in succession to drilling. This has posed the inconvenience of requiring facilities and time for replacement of tools. Thus, a proposal has recently been made, as in Japanese Utility Model Publication No. 25782/93, for a tap for a cored hole which has a guide portion connected to a front end of a tapping portion, and a cutting edge capable of face cutting which is provided at a front end of the guide portion so that the processing of a prepared hole and tapping can be performed by a single step.
With the above-mentioned conventional tap for a cored hole, an incomplete thread portion of the tap increases by the amount of the guide portion. Thus, if the prepared hole for an internal thread is a through hole or has enough space, no problem is posed. However, if the prepared hole for an internal thread is a blind hole or the like and does not have enough space, tapping cannot be performed. Even if tapping is possible because the prepared hole for an internal thread is a through hole or has enough space, an inner peripheral surface of the prepared hole worked by the tapping portion becomes, as is, a crest surface of an internal thread. For this reason or else, sufficiently satisfactory finished surface roughness or accuracy is not always obtained depending on the machining conditions.
The present invention has been accomplished in view of the above-described circumstances. It is an object of the invention to provide taps of a cutting type and a plastic working type which can perform processing of a prepared hole, tapping, and finishing of a minor diameter by a single step, and which can carry out tapping even if a prepared hole for an internal thread is a blind hole or the like and does not have enough space.
A first aspect of the invention for attaining the above object is a tap for performing processing of a prepared hole and tapping by a single step, the tap having a threaded portion for performing tapping by chipping, the threaded portion including a lead whose thread diameter gradually decreases toward a front end of the lead, a complete thread portion continued from the lead, and a plurality of flutes arranged in a circumferential direction; a first cutting edge portion formed at a front end surface of the lead for processing a prepared hole; and a second cutting edge portion formed in a part of an outer periphery of the complete thread portion for finishing a minor diameter of an internal thread, the second cutting edge portion being one or more pitches long.
A second aspect of the invention is a tap for performing processing of a prepared hole and tapping by a single step, the tap having a threaded portion for performing tapping by plastic working, the threaded portion including a lead forwardly tapered in a complete thread form, a parallel thread portion continued from the lead, and a plurality of oil grooves arranged in a circumferential direction; a first cutting edge portion formed at a front end surface of the lead for processing a prepared hole; and a second cutting edge portion formed in a part of an outer periphery of the lead for finishing a minor diameter of an internal thread.
According to the tap of the present invention, processing of a prepared hole, tapping, and finishing of a minor diameter can be performed by a single step without replacement of tools, the processing man-hours and the processing time can be markedly decreased, and productivity can be increased. Furthermore, the first cutting edge portion is directly provided at the front end surface of the lead of the threaded portion, without a guide portion provided there, and the second cutting edge portion is provided in the threaded portion. Thus, the entire length of the tap need not be increased, and tapping can be performed even if a prepared hole for an internal thread is a blind hole or the like and does not have enough space.